Particular embodiments generally relate to voltage controlled oscillators (VCOs).
Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
FIG. 1 depicts a conventional voltage controlled oscillator 100. An inductor/capacitor tank (LC tank) 101 is formed by a parallel or series connection of an inductor 102 and a capacitor 104. LC-tank 101 is coupled to an active circuit, which is represented as a cross-coupled transistor pair 106. As shown, cross-coupled transistor pair 106 is coupled in parallel to LC-tank 101 and includes a first transistor 108a (M1) and a second transistor 108b (M2).
In operation, for a resonant frequency, the impedance of LC-tank 100 becomes infinite and when energy is stored initially in the tank, it circulates from voltage energy in capacitor 104 to current energy in inductor 102, and vice versa. This exchange of energy occurs at the resonant frequency, with the voltage and current being sinusoidal in quadrature phase with respect to each other and the ratio of the voltage and current amplitude being:V/I=√{square root over (LC)}. 
Reactive components, such as inductor 102 and capacitor 104, have losses in the real world implementation. The losses may be modeled as series or parallel resistances to LC-tank 100. The losses may dampen the oscillating signal generated by LC-tank 100. The active circuit may be used to compensate for the losses.
A negative resistance is synthesized by cross coupled transistor pair 106 and is explained by describing the currents sourced/sinked by cross-coupled transistor pair 106 to/away from LC-tank 101. The current sourced/sinked is biased by a current source (Ibias) 110. When a voltage at a node Vp is at its positive peak value, the resistance of LC-tank 101 is taking away current from node Vp. To compensate for this, transistor 108a is sourcing current into node Vp. When the voltage at node Vp is at its negative peak value, the resistance of LC-tank 101 is sourcing current into node Vp and transistor 108a is sinking current from node Vp. The dual behavior happens at node Vn.
Cross-coupled transistor pair 106 is behaving as a negative resistance because cross-coupled transistor pair 106 is sourcing current from nodes Vp or Vn when the voltage is at a maximum at the nodes and sinking current from nodes Vp or Vn when the voltage is at a minimum at the nodes. The ratio between the voltage at nodes Vp or Vn to the current flowing out of nodes Vp or Vn is negative. Synthesizing the negative resistance sustains the oscillation at a desired frequency.
VCO 100 may be used in a radio frequency (RF) transceiver. VCO 100 offers advantages in that it is simple and offers relatively good performance. However, in some more advanced RF applications, voltage controlled oscillators with a better phase noise (higher purity) may be required. Typically, VCO 100 may be run with a higher current or through the use of an external inductor with a higher quality factor (Q).
The above solutions may reduce phase noise, but still inject current (energy) alternatingly to one side of VCO 100 to replenish the energy loss of LC-tank 101. The current energy injected into LC-tank 101 alternates essentially around the time when the oscillating signal changes polarity or crosses a middle point. Injecting current causes transistors 108a and 108b to alternatingly be on during the zero crossing point of the oscillating signal. While this will maintain the oscillation of VCO 100, some RF designs may still require VCOs with a better phase noise.